This invention relates to self-dispersing pigments and particularly to a process of making such pigments and to the use thereof in ink jet inks.
Aqueous dispersions of pigments are known in the art and have been used in various applications, such as, inks for printing (particularly ink jet printing); waterborne paints and other coating formulations for vehicles, buildings, road markings and the like; cosmetics; pharmaceutical preparations; etc. Because pigments are typically not soluble in an aqueous vehicle, it is often required to use dispersing agents, such as, polymeric dispersants or surfactants, to produce a stable dispersion of the pigment in the vehicle.
The use of dispersing agents, however, increases the viscosity of the dispersion over what it would be without the dispersing agents. The increase in viscosity, while not particularly a problem for some uses, presents a significant disadvantage in others. For example, viscosity limitations on the end use application of the dispersion (e.g., ink jet inks) may be exceeded if binders are added to improve properties of the final product. Thus, the viscosity of the dispersion itself will reduce the formulation latitude for final products.
Self-dispersing pigments, particularly self-dispersing carbon black pigments, have been known in the art for many years. For example, U.S. Pat. No. 2,439,442 discloses a process in which a carbon black pigment is exothermically reacted with a water solution of sodium hypochlorite, or is subjecting the to electrolysis in a sodium chloride solution, or is suspended in a sodium hydroxide solution and treated with chlorine gas to alter the colloidal properties such that the carbon black will readily and spontaneously disperse in water. Inks made from these dispersions are said to be waterfast on newsprint.
Donnet et al., Étude de l' action chimique des oxydants sur le noir de carbhone, École Supérieure de Chemie de Mulhouse, No. 294 (1962) pp. 1727-1735, provides a comprehensive overview of the effect of oxidants on carbon black to render them more hydrophilic. Oxidants discussed include potassium permanganate, sodium chloride, sodium chlorate, sodium perchlorate, sodium persulfate, nitric acid and sodium hypochlorite.
U.S. Pat. No. 3,023,118 teaches a process of oxidizing carbon black with dilute nitric acid to render it more readily dispersable. U.S. Pat. No. 3,279,935 discusses gas phase oxidation of carbon black generally and, in particular, teaches a gas phase oxidation process in which carbon black is treated with an oxygen containing gas admixed with a peroxide gas. U.S. Pat. No. 3,347,632 teaches a process of making hydrophilic carbon black by reacting the carbon black in an aqueous solution of sodium hypochlorite.
There has been a flurry of activity recently to formulate inks using self-dispersing pigments. For example, U.S. Pat. No. 5,554,739 and WO 96 18688 teach the production of surface modified carbon black pigments in which the carbon black is reacted with diazonium salts. WO 97 47698 teaches treating carbon black with silicone to improve its dispersibility. Inks containing such surface modified carbon black pigments, or the use of such carbon black pigments in inks, have also been proposed in WO 96 18694; U.S. Pat. No. 5,713,988; WO 97 49774; and WO 98 06788. Treatment of carbon black pigment with an aqueous solution of a hypohalite (e.g., sodium hypochlorite) and its uses are proposed in U.S. Pat. No. 5,609,671; U.S. Pat. No. 5,718,746; EP 0 802 247 and EP 0 819 737.
All of these treatment processes in the prior art have one disadvantage or another. The gas phase oxidation can be very exothermic and thus pose a significant safety hazard. Both the gas and wet phase processes generate significant amounts of by-products which need to be removed through purification steps. To overcome these disadvantages, EP 0 896 986 discloses a process of treating carbon black with ozone in the presence of water and inks and coatings made therefrom. The carbon black produced by such a process are said to have less contaminants than prior art processes and produce more stable dispersions. Even these pigments, however, are not of sufficient quality to be commercially viable for use in demanding applications, such as, ink jet printing.